All viruses in the Moose family infect program files by appending the viral code to the original file. Different variants infect different types of files, either COM, EXE or SYS. When the virus infects a SYS file it overwrites the file's header information. Infected SYS files crash the computer.
Moose viruses are not memory-resident. They only infect files when run. The virus tries to find a suitable program in the current directory, and if there are no files to infect, it switches one level up and tries again. It repeates this until it reaches the root directory of the drive.
When a suitable file is found, Moose appends its code in the file and modifies the entry point of the file. Besides this Moose may change one byte somewhere in the host file.
The result of this one-byte corruption is always unknown. Usually it does not affect the program in any way but sometimes it causes the program to crash at runtime. The virus uses the computer's real time clock to decide whether to corrupt a byte in the host file or not. In most cases the corrupted program is not repairable. It has to be restored from a backup copy or the original setup media.
The Moose viruses were found in Gothenburg, Sweden in 1993. They were discovered at the local university which may mean that the author was a student at the university.